1. Field of Invention
This invention is directed to semiconductor devices that have reduced thermal resistance.
2. Description of Related Art
Some semiconductor devices experience poor performance because the heat that is generated during operation is not able to flow out of the semiconductor device efficiently. This leads to an increase in temperature that is detrimental to the performance of the semiconductor device. In many cases the temperature rise (ΔT) is proportional to the heat per unit time (ΔW) that is generated by the semiconductor device, i.e., ΔT=RΔW. Here, the proportionality factor R is the thermal resistance of the semiconductor device.
The inefficient heat flow out of the semiconductor device is often attributable to those parts of the device that have low thermal conductivity. In some semiconductor devices, during operation, heat must flow from the point where the heat is generated to an external heat sink. The heat sink has a sufficiently large thermal mass so that its temperature remains equal to the ambient air. However, to reach the heat sink, the heat must often flow through a region of the device that has a low thermal conductivity. In this case, the thermal resistance R of the semiconductor device will be high. As a result, the temperature in the semiconductor device will be much higher than the temperature of the heat sink.
In light emitting diodes and lasers, heat is generated in the active region of the device and in the p- and n-contacts. This heat must usually flow through the substrate to reach the external heat sink. Light emitting diodes and lasers can be formed on substrates that have poor thermal conductivities. Sapphire, a commonly used substrate, has, for example, a thermal conductivity (Kth) of 0.42 W/cmK at room temperature.
The light output intensity of a light emitting device depends on the temperature at which the light emitting device operates. With a constant current flowing through such a light emitting device, the light output intensity is reduced as the temperature increases. In some cases, high temperatures will prevent lasing in laser diodes and the like. In semiconductor devices that have two or more light emitting devices adjacent to each other, the light ouput intensity of the first light emitting device is affected by the output power of the adjacent light emitting devices. This occurs because the temperature in the first device is affected by the amount of dissipated power, and therefore the amount of heat, that is generated by the adjacent devices. This effect is known as thermal cross-talk. For many applications, e.g., laser printing, cross-talk between adjacent light emitting devices is highly undesirable, because the light emitting devices are desirably separately addressable and completely independent from each other.